use uniform permission checks for all mount propagation changes

use uniform permission checks for all mount propagation changes

do_change_type() and do_set_group() are operating on different
aspects of the same thing - propagation graph.  The latter
asks for mounts involved to be mounted in namespace(s) the caller
has CAP_SYS_ADMIN for.  The former is a mess - originally it
didn't even check that mount *is* mounted.  That got fixed,
but the resulting check turns out to be too strict for userland -
in effect, we check that mount is in our namespace, having already
checked that we have CAP_SYS_ADMIN there.

What we really need (in both cases) is
	* only touch mounts that are mounted.  That's a must-have
constraint - data corruption happens if it get violated.
	* don't allow to mess with a namespace unless you already
have enough permissions to do so (i.e. CAP_SYS_ADMIN in its userns).

That's an equivalent of what do_set_group() does; let's extract that
into a helper (may_change_propagation()) and use it in both
do_set_group() and do_change_type().

Fixes: 12f147ddd6de "do_change_type(): refuse to operate on unmounted/not ours mounts"
Acked-by: Andrei Vagin <avagin@gmail.com>
Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Tested-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>